Try to update cmake on Mac travis.

travis_cmake_mac.sh

@@ -1,6 +1,9 @@
 #!/bin/bash
-which cmake
 brew uninstall cmake --force
 brew cleanup
-which cmake
+wget https://cmake.org/files/v3.12/cmake-3.12.0-rc2-Darwin-x86_64.tar.gz
+tar -xvzf cmake-3.12.0-rc2-Darwin-x86_64.tar.gz
+cp -r cmake-3.12.0-rc2-Darwin-x86_64/CMake.app /Applications
 sysctl -n hw.physicalcpu
+which cmake
+cmake --version